The continuing increases in the functional capacity of integrated circuits (ICs) over the last few years have been both astounding and beneficial. However, accompanying these increases are attendant technical problems that demand creative solutions. One such problem has been the increase in input/output (I/O) pads that typically result from increases in the amount of circuitry that can be incorporated onto an IC. The number of I/O pads on a traditional wire-bonded IC, which involves bonding wires from the I/O pads of the IC die to the substrate, is generally limited by the length of the IC perimeter because such I/O pads typically reside at the edges of the IC. Thus, reductions in the size of transistors and other electronic devices incorporated on a single die generally create a need for more I/O pads than what traditional wire-bonding technology can offer.
To satisfy this need, alternatives to wire-bonding techniques have been devised to increase the overall interconnection density of ICs. One such alternative is the xe2x80x9cflip chip,xe2x80x9d which utilizes I/O connections across the top surface of the die. Thus, the connections are not restricted to the perimeter of the IC. Typically, solder xe2x80x9cbumpsxe2x80x9d are formed on these connections. The solder bumps are then covered with solder flux, the die is flipped over so that the bumps make contact with the connection points of the IC substrate, and the die-substrate assembly is heated to reflow the solder. Hence, the necessary electrical contacts between the die and substrate are made by way of the solder bumps with the aid of the solder flux. FIG. 1 is a simplified perspective view of a typical flip chip assembly 100, with a die 110 connected to a substrate 120 by way of solder bumps 130, with die 110 and substrate 120 defining a narrow, substantially planar space 140 therebetween.
Tests on flip chip devices have shown that repeated heating and cooling of the IC during normal use tends to place sufficient thermal stress on the integrated circuit (die-substrate) assembly to cause some of the connections made via solder bumps 130 to break, creating electrical discontinuities between die 110 and substrate 120. To prevent such breaks, an underfill material (generally an adhesive) is normally employed to fill planar space 140 to maintain the structural integrity of the assembly and prevent the electrical connections from breaking. However, after the solder reflow, some flux residue remains in planar space 140 that must be removed by way of an IC cleaning solution before the underfill can be applied. The cleaning process is vital since leftover residue within planar space 140 prevents the underfill from reaching the entirety of planar space 140, thus adversely affecting the structural integrity and overall reliability of flip chip assembly 100.
Complete cleaning of the flux residue from planar space 140 of flip chip assembly 100 has proven to be rather difficult. The distance between die 110 and substrate 120 is normally quite narrow, on the order of 70 um or less. Further complicating the process is the fact that several rows of solder bumps 130 may exist in planar space 140, thus making access to all of planar space 140 even more problematic.
Currently, IC assemblies are normally cleaned using commercially available centrifugal cleaners and cleaning solutions. As shown in a simplified manner in FIG. 2, a centrifugal cleaner 200 employs a tank 210 that is filled with an IC cleaning solution 220 during the cleaning process. Centrifugal cleaner 200 usually holds several IC assemblies, such as flip chip assembly 100, using a cleaning fixture 230 immersed in cleaning solution 220 inside tank 210. A tank-filling mechanism (not shown) of centrifugal cleaner 200 is used to fill tank 210 with IC cleaning solution 220. Cleaning fixture 230 is then spun or agitated on a central vertical axis in cleaning solution 220 by way of a motor 240. Cleaning solution 220 is then drained from tank 210, and water rinse and spin-drying cycles in centrifugal cleaner 200 then normally follow. Cleaning fixture 230 holds several flip chip assemblies 100, or other similar IC assemblies, horizontally within IC cleaning solution 220.
Cleaning fixture 230 may be implemented in a variety of ways. For example, cleaning fixture 230 may consist of a central carousel to which one or more cassettes are attached. Each carousel would then be loaded manually with flip chip assemblies 100 prior to the cleaning process. Also, flip chip assemblies 100 may be held in boats 300 (FIG. 3), each of which holds several flip chip assemblies 100 throughout a majority of the IC manufacturing process. In that case, a cleaning fixture holds several such boats 300 containing flip chip assemblies 100 to be cleaned. Other methods of implementing cleaning fixture 230 not disclosed herein are also employed in the industry.
Unfortunately, as displayed in FIG. 4, which shows a top view of flip chip assembly 100 after being agitated or spun in a bath of cleaning solution 220 in centrifugal cleaner 200, tests have shown that cleaning solution 220 almost always fails to penetrate the entirety of planar space 140 (not shown explicitly in FIG. 4) between die 110 and substrate 120, leaving some flux residue behind because an air pocket 400 becomes trapped in planar space 140. When flip chip assembly 100 is positioned horizontally, cleaning solution 220 encroaches from all sides of planar space 140 simultaneously, trapping air pocket 400 approximately in the center of planar space 140. Air pocket 400 then acts as a countervailing force against the entry of cleaning solution 220 into planar space 140. Cleaning solution 220 is thus prevented from reaching all of planar space 140, allowing some of the flux residue from the solder reflow phase to remain. The remaining flux residue thus prohibits the underfill material subsequently applied from occupying all of planar space 140. Tests also confirm that no amount of spinning or agitation in cleaning solution 220 will force air pocket 400 from planar space 140 so that cleaning solution 220 may occupy all of planar space 140.
To remedy this problem, the use of a apparatus and method of cleaning the tight spaces in integrated circuit assemblies, such as, for example, between the die and substrate of a flip-chip IC, that would result in the complete removal of the flux residue in the space would be advantageous. Without any flux residue present in the planar space, the underfill material to be applied for purposes of structural integrity may fill all of the space, thus preventing the breakage of the various connections between the substrate and die. The cleaning of other types of integrated circuit assemblies involving similar tight spaces, such as, for example, ball grid arrays (BGAs) and direct chip attach (DCA) assemblies, whereby a die is attached directly to a printed circuit board (PCB), would also benefit from such an apparatus and method.
Specific embodiments according to the present invention, to be described herein, provide an effective way of cleaning a space within an integrated circuit assembly without trapping air inside the space. For example, one embodiment of the invention provides a method of cleaning an IC assembly, such as a flip chip IC. To allow the cleaning solution to enter the space without trapping an air pocket inside, the IC assembly is held at an incline from horizontal. The IC assembly is then immersed slowly in the cleaning solution so that the space is completely filled with the cleaning solution prior to the integrated circuit assembly becoming completely submerged within the solution. Since the cleaning solution fills all of the space, all flux residue will be dissolved, allowing the underfill material used later in the IC manufacturing process to fill the entire space, helping to create a structurally reliable IC assembly.
Another embodiment of the invention involves an IC cleaning apparatus that holds an IC assembly at an incline from horizontal. The IC assembly is usually retained either directly or indirectly by a cleaning fixture. The cleaning apparatus then slowly immerses the IC assembly in the cleaning solution bath so that the cleaning solution completely fills the space, thereby allowing air in the space to escape prior to total submergence of the IC assembly in the solution.
Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.